The sampling of fluids contained in subsurface earth formations provides a method of testing formation zones of possible interest by recovering a sample of any formation fluids present for later analysis in a laboratory environment while causing a minimum of damage to the tested formations. The formation sample is essentially a point test of the possible productivity of subsurface earth formations. Additionally, a continuous record of the control and sequence of events during the test is made at the surface. From this record, valuable formation pressure and permeability data as well as data determinative of fluid compressibility, density and relative viscosity can be obtained for formation reservoir analysis.
Early formation fluid sampling instruments such as the one described in U.S. Pat. No. 2,674,313 were not fully successful as a commercial service because they were limited to a single test on each trip into the borehole. Later instruments were suitable for multiple testing; however, the success of these testers depended to some extent on the characteristics of the particular formations to be tested. For example, where earth formations were unconsolidated, a different sampling apparatus was required than in the case of consolidated formations.
Down-hole multi-tester instruments have been developed with extensible sampling probes for engaging the borehole wall at the formation of interest for withdrawing fluid samples therefrom and measuring pressure. In downhole instruments of this nature it is typical to provide an internal draw-down piston which is reciprocated hydraulically or electrically to increase the internal volume of a fluid receiving chamber within the instrument after engaging the borehole wall. This action reduces the pressure at the instrument formation interface causing fluid to flow from the formation into the fluid receiving chamber of the tool. Heretofore, the pistons accomplish suction activity only while moving in one direction. On the return stroke the piston simply discharges the formation fluid sample through the same opening through which it was drawn and thus provides no pumping activity. Additionally, unidirectional piston pumping systems of this nature are capable of moving the fluid being pumped in only one direction and thus causes the sampling system to be relatively slow in operation.
Early down-hole multi-tester instruments were not provided with a capacity for substantially continuous pumping of formation fluid. Even large capacity tools have heretofore been limited to a maximum draw-down collection capability of only about 1000 cc and they have not heretofore had the capability of selectively pumping various fluids to and from the formation, to and from the borehole, from the borehole to the formation, or from the formation to the borehole. U.S. Pat. No. 4,513,612 describes a Multiple Flow Rate Formation Testing Device and Method which allows the relatively small volume draw-down volume to be discharged into the wellbore or to be forced back into the formation. The use of "passive" valves as taught in this method precludes reverse flow. This method does provide for limited or one shot reverse flow much like a hypodermic needle but transferring large volumes of fluid between two reservoirs in a near continuous manner is not achievable with this method. It is desirable, therefore, to provide a down-hole fluid sampling tool with enhanced pumping capability with an unlimited capacity for discharge of formation fluid into the wellbore and with the capability to achieve bi-directional fluid pumping to enable a reverse flow activity that permits fluid to be transferred to or from a formation. It is also desirable to provide a down-hole testing instrument having the capability of selectively pumping differing fluids such as formation fluid, known oils, known water, known mixtures of oil and water, known gas-liquid mixtures, and/or completion fluid to thereby permit in situ determination of formation permeability, relative permeability and relative viscosity and to verify the effect of a selected formation treatment fluid on the producibility of connate fluid present in the formation.
In all cases known heretofore, down-hole multi-test sampling apparatus incorporates a fluid circuit for the sampling system which requires the connate fluid extracted from the formation, together with any foreign matter such as fine sand, rocks, mud-cake, etc. encountered by the sampling probe, to be drawn into a relatively small volume chamber and which is discharged into the borehole when the tool is closed as in U.S. Pat. No. 4,416,152. Before closing, a sample can be allowed to flow into a sample tank through as separate but parallel circuit. Other methods provide for the sample to be collected through the same fluid circuit.
U.S. Pat. No. 3,813,936 describes a "valve member 55" in column 11, lines 10-25 which forces trapped wellbore fluids in a "reverse flow" through a screen member as the "valve member 55" is retracted. This limited volume reverse flow is intended to clean the screen member and is not comparable to bi-directional flow described in this disclosure because of the limited volume.
Mud filtrate is forced into the formation during the drilling process. This filtrate must be flushed out of the formation before a true, uncontaminated sample of the connate fluid can be collected. Prior art sampling devices have a first sample tank to collect filtrate and a second to collect connate fluid. The problem with this procedure is that the volume of filtrate to be removed is not known. For this reason it is desirable to pump formation fluid that is contaminated with filtrate from the formation until uncontaminated connate fluid can be identified and produced. Conventional down-hole testing instruments do not have an unlimited fluid pumping capability and therefore cannot ensure complete flushing of the filtrate contaminant prior to sampling.
Estimates of formation permeability are routinely made from the pressure change produced with one or more draw-down piston. These analyses require that the viscosity of the fluid flowing during pumping be known. This is best achieved by injecting a fluid of known viscosity from the tool into the formation and comparing its viscosity with recovered formation fluid. The permeability determined in this manner can then be reliably compared to the formations in off-site wells to optimize recovery of fluid.
A reversible pump direction will also allow a known fluid to be injected from the tool or borehole into the formation. For example, treatment fluid stored within an internal tank or compartment of the instrument or drawn from the wellbore may be injected into the formation. After injection, additional draw-downs and/or sampling may take place to determine the effect of the treatment or completion fluid on the producibility of the formation. Early formation sampling instruments have not been provided with features to determine the optimum sampling pressures. The present invention also provides a positive method for overcoming differential sticking of the packer by pumping fluid into the formation at a high pressure thereby unseating the packer.
The present invention overcomes the deficiencies of the prior art by providing method and apparatus for achieving in situ pressure, volume and temperature (PVT) measurement through utilization of a double-acting, bi-directional fluid control system incorporating a double-acting bi-directional piston pump capable of achieving pumping activity at each direction of its stroke and capable through valve stroke to achieve bi-directional fluid flow and having the capability of selectively discharging acquired connate fluid into the wellbore or into sample containing vessels or pumping fluid from the wellbore or a sample containing vessel into the formation. The connate fluid samples are acquired in such manner that the sample does not undergo phase separation at any point in the sample acquisition process.